Method of producing crystals.



H. H. DOW.

METHOD OF PRODUCING CRYSTALS.

APPLICATION FILED NOV.11, 1907.

I 172 velzzoz' Patented Oct. 3, 1911.

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HERBERT H. DOW, OF MIDLAND, MICHIGAN, ASSIGNOR TO THE DOW CHEMICAL COM-PANY, OF MIDLAND, MICHIGAN, A CORPORATION OF MICHIGAN.

METHOD OF PRODUCING CRYSTALS.

Patented Oct. 3, 1911. Serial No. 401,580.

My present invention relates to the mann facture of; chemical productsof a crystalline nature and finds a convenient exemplification in themanufacture of common salt (NaCl). It is not intended, however, by thusdescribing such process in the particular connection named to imply alimitation thereof-to any restricted field, for as will be obviouscrystals of many salts may be advantageously formed by the employment ofmy method.

In the manufacture of salt by the pan process, as well as in grainers,which are largely used 1n Mlchigan and in western New York, the brine ismore or less purified'before evaporation and is supplied to the pan orgrainer, as the case may be, in just suflicient quantities to replacethe water evaporated. The evaporation is then continued until themother-liquors become so highly charged with calcium and magnesiumchlorid that the resulting product is unmarketable. Even where theevaporation is not continued to a point where the impurities named arecaused to crystallize out along with the salt, it being of courseunderstood that the latter in a solution of-ordinary concentration willdeposit first, the methods of manufacture above described areobjectionable unless the preliminary purification of the brine is quitethorough, This for the reason that the salt crystals as deposited, eventhough themselves pure, will carry with them a film of the liquorconstituting the impure concentrate or mother-liquor and this upondrying leaves a thin coating upon the crystal of the hygroscopicdeliquescent calcium salts. These cause the granules to cohere and ingeneral depreciate the value of the product. It is with a view torendering easily possible the production of salt of a degree of puritysuch as will avoid the dlfficulty noted,

that of attracting moisture, that the present process has been devised.

lVith this and other ends'in view said invention then consists of thesteps hereinafter fully described and particularly pointed out in theclaims.

The annexed drawing and the following description set forth in detailone approved method of carrying out the invention, such disclosed mode,however, constituting but one of various ways in which the principleofthe invention may be used.

Insaid annexed drawing: the single figure there appearing illustrates inmore or less diagrammatic form a modification of an apparatus for themanufacture of salt by the grainer process, whereby the same is adaptedfor use in my improved method of producing crystals.

In said annexed drawing A, A, A designate respectively, the members of aseries of open grainers, or evaporators. These, however, instead ofbeing operated as individual evaporating pans, are connected by means ofpipes a so as to permit the flow of the brine being evaporated from onepan to the other in succession, such brine being ad mitted through aninlet pipe a and the final concentrate or mother-liquor being dischargedthrough an outlet pipe o The evaporation of the brine in each of theseveral grainers is effected by means of a steam coil a in the usualfashion, the steam connections for such coils being omitted in thefigure under consideration. In addition to such heating or evaporatingapparatus each pan is provided with means for transferring the salttherefrom to the adjacent pan. To this end a corresponding side a ofeach pan is inclined outwardly so as to approach more orless closely theadjacent Wall a of the neighboring pan and a sloping platform a isprovided leading from the end of such inclined wall of the first pan tothe second pan. Passing around suitable rollers or sheaves a, as thecase may be, is an endless conveyer a preferably constructedv of chainsor cables bearing transverse bars constituting scrapers. By movement ofsuch conveyers in the proper direction, the scrapers will serve to carrythe salt formed in the first pan A up the inclined side a of such panand thence onto the sloping platform a leading into the next pan. Uponits discharge thus into said accordingly be 'seen that the brineentering through inlet pipe at is passed in succession through the wholeseries of evaporating of any adhering the term pan being here employedin a general sense. In the first stage of evaporation of such brine,viz. in pan A the solution will obviously be relatively pure; as it,however, passes into the intermediate pan A and thence into the last panA of the series, taken in this direction, it becomes more pans,

and more heavily charged with impurities owing to the increasedconcentration of the liquor. The crystals, however, that are formed fromsuch most highly concentrated solution are removed by the conveyeroperating in such pan and transferred to pan A, opportunity beingaflorded for the draining of the granular mass as it is carried up theinclined side of the pan by the conveyer. Upon its discharge into thesecond pan, as has been explained, such mass of salt is carried forwardthrough the solution in that pan thus washing the crystals film of theimpure motherliquor carried over from the first pan; and, at the sametime, owing to the simultaneous evaporation of the solution in thesecond pan, an opportunity is provided for a further deposit of saltupon the crystals thus transferred and passing through such pan. Thisoperation is again repeated upon the transfer of the crystals from thispan to the last of the series, any adhering solution from the second panbeing washed as before and another opportunity for crystal growthafforded.

It is obviously immaterial whether or not evaporation in the successivepans be carried on at such a rate as to cause the formation of newcrystals in additon to those passing through. The effect of thetreatment in either event accorded such last named crystals will be toremove the film of adhering impurity and in place thereof to add anadditional coat of the desired product or salt.

It has been stated that the apparatus shown and described herein ismerely illus trative of the application of my improved process offorming crystals to the manufacture of one particular product and alsoto a more or less specific form of apparatus emsuch manufacture. It isnot necployed in here enumerate other fields in essary to which theprocess for usefulness. Attention may be called, however, to the factthat the type of evaporator is quite immaterial since, as has alreadybeen indicated, not only can the ordinary pan be employed in place ofthe grainers shown but various types of vacuum apparatus may likewise beadapted for use in this process. The evaporation may be carried onslowly or vigorously as desired. The mechanical means used for passingthe precipitated crystals through the pans in a direction from impure toless impure liquor may be of any desired form. Again the evaporators maybe merged into one pan properly divided so as to present a series ofcompartments adapted to permit the crystal liquor to flow through themserially in one direction while the crystals are moved in a counterdirection through the same series of comparments by suitable mechanicalmeans.

Other modes of applying the principle of. my invention may be employedinstead of the ones explained, change being made as regards the processherein disclosed, provided the step or steps stated by any one of thefollowing claims or the equivalents of such stated step or steps beemployed.

1 therefore particularly point out and distinctly claim as myinvention 1. In a method of producing crystals from an impure solution,the steps which consist in washing crystals formed from such solution inanother purer solution, and simultaneously evaporating such last namedsolution so as to permit a further production of such crystals.

2. The method of producing crystals from an impure solution, whichconsists-in partially evaporating such solution in one pan, continuingsuch evaporation in a. second pan, the crystals formed in such secondpan to the said first pan.

3. The method of producing crystals from an impure solution, whichconsists in passing such solution in succession through a series ofevaporatin pans, and simultaneously transferring t e crystals from panto to that in which the solution flows.-

4:. The method of producing crystals from an impure solution, whichconsists in evaporating such solution in successive stages, andtransferring the crystals formed from the solution in a more highlyimpure stage to such solution in a less impure stage.

5. The method of producing crystals from an impure solution, whichconsists in evaporating such solution stages, and transferring thecrystals formed from the solution in its most highly impure.

would find an opportunity and simultaneously transferring in successivestage in succession through the solution in stages of graduallydecreasing concentration.

6. In a method of producing crystals from an impure solution, the stepswhich consist in conducting the evaporation in two or more successivestages, and mechanically moving the precipitated crystals through thesaid stages in a direction passing from a late stage of evaporationtoward and through the earlier stages whereby the crystals formed in thelater stages are washed in the purer liquor of earlier stages whileevaporation is progressing therein.

7. The method of producing crystals from an impure solution, whichconsists in evaporating such solution in stages of successively greaterimpurity, and simultaneously continuously moving the crystals formed inany particular stage of the series, together with those previously addedfrom a less pure stage, through such particular stage and into the nextpurer one.

8. The method of producing salt crystals from an impure brine, whichconsists in evaporating such brine in stages of successively greaterimpurity, and simultaneously continuously moving the crystals formed inany particular stage, together with those previously added from a lesspure stage, through such particular stage and into the next purer one.

Signed by me this 31 day of October,

, HERBERT H. DOW. Attested by THos. GRIsWoLD, Jr., C. A. SAUNDERS.

